<i>SPITZER</i>24 μm IMAGES OF PLANETARY NEBULAE

Abstract

Spitzer MIPS 24 µm images were obtained for 36 Galactic planetary nebulae (PNe) whose central stars are hot white dwarfs (WDs) or pre-WDs with effective temperatures of ∼100,000 K or higher. Diffuse 24 µm emission is detected in 28 of these PNe. The eight non-detections are angularly large PNe with very low Hα surface brightnesses. We find three types of correspondence between the 24 µm emission and Hα line emission of these PNe: six show 24 µm emission more extended than Hα emission, nine have a similar extent at 24 µm and Hα, and 13 show diffuse 24 µm emission near the center of the Hα shell. The sizes and surface brightnesses of these three groups of PNe and the non-detections suggest an evolutionary sequence, with the youngest ones being brightest and the most evolved ones undetected. The 24 µm band emission from these PNe is attributed to [O IV] 25.9 µm and [Ne V] 24.3 µm line emission and dust continuum emission, but the relative contributions of these three components depend on the temperature of the central star and the distribution of gas and dust in the nebula. 1